1. Field of the Invention
The invention relates to an intra-aortic balloon pump system. More particularly, the invention relates to a method and device for accurately measuring arterial blood pressure via a pressure sensor on an intra-aortic balloon catheter.
2. Description of the Prior Art
A key function of many catheters is that of continuously monitoring blood pressure. In many cases, this process must provide accurate measurement of the blood pressure waveform's high frequency components. For example, reliable detection of the dicrotic notch of the aortic blood pressure waveform typically requires a pressure signal having a bandwidth of 15 Hz or better. Detection of the dicrotic notch is generally used for the inflation/deflation timing of an intra-aortic balloon (“TAB”) catheter.
Conventional TAB catheter invasive pressure monitoring is performed with low cost piezo resistive transducers, which are hydraulically coupled to the targeted monitoring site in the patient. Typically, the tip of the TAB is hydraulically coupled to a transducer external to the patient's body.
The chief benefit of the current configuration is its low cost. The typical disposable monitoring kit, inclusive of all tubing, a continuous flush device, and a pre-calibrated transducer is very affordable.
Unfortunately, hydraulically coupled transducers are vulnerable to motion artifact. Inflation and deflation of the IAB balloon membrane causes movement of the IAB catheter, which generates inertial forces in the fluid and pressure transducer coupled to it, resulting in artifact. Patient movement can cause a similar result. Therefore, a waveform may contain artifact in the form of “overdamping”, “ringing”, and “catheter whip”.
Another disadvantage of the traditional transducer configuration is that it requires the use of a separate inner lumen to couple the transducer to the tip of the IAB catheter. Traditionally, IAB catheters have two lumens in the catheter: a gas shuttle lumen and an inner guidewire lumen. The gas shuttle lumen has to be large enough to allow the gas to shuttle back and forth without undue restriction to ensure rapid inflation and deflation of the IAB membrane. The guidewire lumen serves two functions: It hosts the guidewire for IAB placement and, once the IAB is placed and the guidewire is removed, it is used for pressure monitoring. When the guidewire lumen is present, it reduces the cross-sectional area available for shuttle gas flow and, hence, it indirectly slows IAB inflation and deflation. Therefore, it is desirable to eliminate the guidewire lumen altogether. Elimination of the guidewire lumen creates a need for an alternate means for measuring arterial pressure.
One alternate means for monitoring blood pressure involves the use of a fiber optic sensor as disclosed in U.S. patent application Ser. Nos. 09/734,755 and 09/925,143, filed on Dec. 11, 2000 and Aug. 9, 2001, respectively, and assigned to Datascope Investment Corp., herein incorporated by reference in their entirety.
Alternatively one can use a micromanometer on the tip of the IAB, such as marketed by companies such as Millar, Endosonics, and Radi. See U.S. Pat. Nos. 5,431,628, 5,902,248 and 6,398,738, all herein incorporated by reference. These devices have an excellent frequency response, with system bandwidths greater that 200 Hz. They are not subject to the negative effects of hydraulic coupling, such as motion artifact, air bubbles, and catheter whip. Also, they retain good performance in the presence of small blood clots. Attempts have been made to use micro-manometers for IAB pump timing, see U.S. Pat. Nos. 3,585,983 and 4,733,652, herein incorporated by reference. These attempts have proven to be unsuccessful because the device is prone to signal drift and is susceptible to interference from electro-surgical units.
Unfortunately, both micro-manometers and fiber optic sensors are prone to the problem of signal drift. Many different physical phenomena may cause drift. Changes in the apparent zero offset can occur with any hydraulically coupled pressure sensor simply as a result of changes in the relative heights of the sensor and the point of measurement in the patient. This is a direct result of the hydrostatic pressure exerted by the fluid-filled line upon the sensor, i.e. the “hydraulic head”. In normal practice, care is taken to assure that the point of measurement (such as the patient's right atrium) is at the same level as the pressure sensor.
Drift can also occur as a result of changes within the sensor itself. A typical sensor construction employs a deformable diaphragm which has one side exposed to the medium being measured and the other side exposed to a known reference pressure. This construction is commonly used for various sensor designs, including electronic and fiber optic sensors. In the electronic sensor, the diaphragm's deformation is sensed by measurement of changes in the electrical characteristics of an electrical circuit, which is mechanically coupled to the diaphragm. In the fiber optic sensor, the diaphragm's deformation is sensed optically by fiberoptically-transmitted light, which is retro-reflected by a surface of the diaphragm. In any case, if the reference pressure changes, the apparent pressure as indicated by the sensor will change as well.
In one specific application of the above design approaches, a sealed reference chamber faces the diaphragm. This chamber encloses a small volume of gas, which is at atmospheric pressure at the time of sensor fabrication. The pressure of this gas-filled chamber is the reference pressure. Zero drift, also known as offset, can occur as the temperature of the sensor changes; the gas will either expand as it is warmed, or contract as it is cooled. The zero drift will occur as a result of the consequent change in the diaphragm position.
In another specific application of the above design approaches, the reference pressure is a vacuum (zero pressure), which is maintained in the reference chamber. This construction is less resistant to temperature induced zero drift changes than the previous example, since there is no gas within the chamber to expand or contract.
The sensor's offset can also drift due to various other factors, such as repetitive stress, moisture absorption, chemical reactions, mechanical fatigue, aging, micro-structural changes or temperature-induced stress.
For both of the above constructions, as well as for others, drift in the transducer's scale factor (gain) can also occur. This can be the result of a change in stiffness of the diaphragm, which can occur due to various factors, such as repetitive stress, moisture absorption, chemical reactions, aging, micro-structural changes or temperature changes. In addition, the adhesive bonds which couple the diaphragm to the reference chamber may result in a change in stiffness, causing a change in the sensitivity of the device to pressure. Finally, the interface electronics, which convert the optical signal to an electrical signal can be prone to both gain and offset drift.
To address the offset drift issue, U.S. Pat. No. 5,158,529, issued to Kanai, discloses a method for re-zeroing the micromanometer by using the pressure from a partially filled balloon as it rests in the aorta. The Kanai method for addressing the problem of drift has a significant disadvantage. Namely, Kanai attempts to correct for offset at a defacto calibration point at mean blood pressure, and thus, does not completely eliminate the drift error. Furthermore, Kanai completely ignores the gain shift problem, which may also significantly contribute to signal drift and results in a loss of accuracy.
Kanai's method for reducing zero point drift is also faulty because an inaccurate method is used to compute mean pressure. Kanai's computation uses the peak and valley of the shuttle gas pressure waveform. These features are likely to be corrupted when they are measured, via the IAB membrane, by the shuttle gas transducer. Further, the formulation used by Kanai is a crude approximation; it only valid for saw-tooth shaped pressure waveforms.
U.S. Pat. No. 6,398,738, issued to Millar, discloses a method and system for reconstructing a high-fidelity pressure waveform with a balloon catheter used in a heart. The AC components of the aortic pressure and the mean aortic pressure are detected separately and then added to form a high-fidelity pressure waveform. The AC components are detected by a catheter based pressure sensor. The mean pressure within the balloon is detected by an external pressure sensor.
The Millar device suffers from a number of drawbacks. Firstly, Millar does not correct for errors in the micro-manometer's gain or drift in gain. In Millar's configuration, the micro-manometer's signal is AC coupled. This signal is used as the source for the amplitude of the AC components of the physiologic signal. If the gain is not corrected, then the amplitude of the displayed pulse will be in error.
Further, Millar uses a separate source for the patient's mean pressure, i.e. the “DC component”, of the blood pressure waveform. The AC and DC signals are summed to yield a composite pressure waveform. The fidelity of the pressure waveform can be corrupted if the filters or methods used to extract the AC signals from the micromanometer and the DC (mean pressure) signal from the secondary source are not coordinated.
In Millar, the “DC signal” is “extracted” from a secondary external pressure transducer and the AC component is extracted by “capacitive coupling”. The capacitor implicitly interacts with other impedances, such as the micro-manometer's resistive bridge to form a “defacto” high pass filter. This filter has a “break frequency”, a frequency at which higher frequencies are passed without significant attenuation. Lower frequencies are attenuated in proportion to their distance from the break frequency. The details of the extraction process are not disclosed. However, Millar requires that the process extract only the mean pressure. This implicitly means that all other AC components are eliminated from the “DC” signal. When this signal is then summed with the AC coupled signal from the micromanometer, it is missing some of the original signal content. Specifically, it is missing the attenuated signals that existed below high pass filter's break frequency. As a consequence of this omission, the displayed waveform is distorted.
While the Kanai zeroing method for reducing signal drift or the Millar high-fidelity reconstruction may be suitable for the particular purpose employed, or for general use, they would not be as suitable for the purposes of the present invention as disclosed hereafter.